In general, the term stent is a medical Anglicism in common use to designate a cannula or device of a cylindrical or tubular shape for intraluminal, usually intravascular, use, which is placed inside an anatomical structure or bodily duct in order to keep it permeable or prevent its collapse after dilation, clearing or surgical release. A stent is typically implanted in a blood vessel at the site of a stenosis or intraluminal aneurysm, i.e. using so-called “minimally invasive techniques”, in which the stent is contained in a radially compressed configuration by a sheath or catheter and is delivered using a stent application device or “inserter” into the required site. The inserter may enter the body from a place of access outside the body, such as through the patient's skin or using a technique of incision in which the blood vessel of entry is exposed to minor surgical equipment.
As used in this document, the term stent also refers to grafts, stent-grafts, vena cava filters, expandable structures and similar implantable medical devices, which are radially expandable endoprostheses. Usually they are intravascular implants capable of being implanted transluminally and they are enlarged radially after being inserted percutaneously.
Stents can be implanted in various lumina or vessels in the body, such as in the vascular system, the urinary tract, and bile ducts, among others. Said stents can be used to reinforce blood vessels and prevent restenosis following angioplasty in the vascular system. Stents may be self-expanding, such as nitinol shape memory stents; also they may be mechanically expandable, such as a balloon-expandable stent; or they may be hybrid expandable.
The use of intraluminal stents is very common in various areas of medicine and veterinary practice. There are various designs of stents for intraluminal insertion into blood vessels and other lumina to prevent or reverse their occlusion. In general, three basic categories of stent-type devices are considered to exist, as follows:                heat-expandable devices,        balloon-expandable devices, and        self-expanding devices.        
The present invention refers to self-expanding stent-type devices which, optionally, have the ability to expand by heat, which are inserted into a vessel within the body in a radially compressed form and mechanically change to a radially expanded form. Once the stent is placed in the desired position in the blood vessel, it expands radially, exerting outwards pressure on the internal surface of the wall of the body vessel in which it has been positioned.
Braided stents are manufactured by braiding (interweaving) wires of a thin metallic material according to different braiding patterns. U.S. Pat. No. 6,083,257A describes a method for braiding stents. According to the number of wires, the braiding angle, the nominal radius, the nominal length, and the braiding pattern used, the mechanical properties and density of the resulting stent mesh may vary considerably. The present invention covers both braided and unbraided stents.
In the present document, the term “nominal radius” refers to the radius adopted by the stent when it is left freely outside a vessel or the positioning device and it coincides with the maximum radius when it is released outside the vessel.
In the present document, the term “nominal length” refers to the length adopted by the stent when it is left freely outside a vessel or the positioning device. Therefore, the stent adopts the “nominal length” when it possesses its “nominal radius”.
Stents are often used for the treatment of intracranial aneurysms (IA), a sector in which there are various types of braided stents. One of these types is known as a “Flow Diverter” (FD, its initials in English), it is densely braided and is placed longitudinally along the vessel affected by the aneurysm, and covers the neck of the aneurysm. Alternatively, coarse braided stents are also used as a scaffolding for the protection of the neck of the IA after the positioning of an intravascular coil, as is made known in U.S. Pat. No. 6,010,468A.
Stents are positioned in the desired place using a catheter, in image-guided operations, typically with an interventional X-ray image, with the aid of a contrast marker which shows the location of the vessel lumen and, where appropriate, the aneurysm to be treated. In the case of aneurysms, the catheter is inserted into the body normally through arteries, for example the iliac artery, and is guided to the location of the aneurysm by a neurointerventional radiologist. Said radiologist will select the position at which the distal end of the stent is placed and will gradually unsheathe the stent until it is fully released in the vessel being treated.
Nevertheless, stents present the difficulty that the final length of the stent when it is positioned inside the body is not accurately known in advance and is difficult to predict with the naked eye.
Usually, the estimation of the final length of a stent when placed inside a vessel is made with the naked eye and the stent is assumed to be released in a straight vessel of constant radius. This method provides very poor references in relation to the final length the stent will have in the patient, as most of the vessels are neither straight nor do they have a constant radius.
When a stent is released outside the human body, as mentioned previously, it adopts its nominal radius. However, if this stent is placed inside a vessel with a radius smaller than its nominal radius, the vessel walls prevent the full expansion of the stent and this forces the device to present a configuration with greater length. The fact that the change in the total length of the stent depends on the morphology of the vessel makes it very difficult to predict the final length of said device, prior to its positioning. As the medical practitioner is unable to predict accurately the final length of the stent placed inside the patient, it may happen that collateral branches of the vessel being treated become obstructed or occluded, and this may cause injury to the patient. Furthermore, in the case of intracranial aneurysms, variation in the density of the stent mesh as a result of the various degrees of expansion makes the effect of the device on the blood flow inside the aneurysm difficult to predict. These potentially adverse effects of the treatment mean that it is necessary to create a tool which makes it possible to predict accurately the final length and configuration of the stent, once placed at a particular position inside the lumen of a vascular structure in the body.
There are antecedents which describe methods for modelling stents. Deformable models have been used to simulate the behaviour of a stent when it is positioned inside the lumen of a vessel (Larrabide, I. et al. “Fast virtual deployment of self-expandable stents: method and in vitro evaluation for intracranial aneurysmal stenting”, Medical Image Analysis, 2012, 16(3), 721-730). However, said method does not allow the change in length of the stent to be predicted, as it takes no account of its mechanical behaviour.
Other methods based on mechanical deformation of a structure similar to a cylinder have also been proposed (Cebral, J. R. and Lohner, R. “Efficient simulation of blood flow past complex endovascular devices using an adaptive embedding technique” IEEE Transactions on Medical Imaging, 2005, 24(4), 468-476), but they are not able to predict the change in the length of the stent either.
Recently, a method has been disclosed based on the use of finite elements and a detailed description of the braiding pattern, which allows more accurate modelling of the mechanical behaviour of the stent-type device (Ma, D. et al. “Computer modelling of deployment and mechanical expansion of neurovascular flow diverter in patient-specific intracranial aneurysms” Journal of Biomechanics, 2012, 1-8). This method provides considerable accuracy when it comes to modelling the behaviour of a stent, however, the obtainment of the models is extremely complex and long.
Other methods based on the obtainment of images of lumina of the vessels to be treated and modelling for the determination of the most appropriate stent are made known in International Patent Applications WO2006/093776 and WO2011/038044 and United States Patent Application US2007/0135707.
International Patent Application WO2006/093776 discloses a procedure for modelling stents based on the use of an ultrasound imaging system for obtaining images of blood vessels, detecting defects in said vessels and using said images to perform graphic simulations with various stents to check whether the length and position are appropriate. International Patent Application WO2011/038044, for its part, discloses an automated procedure for simulating the length and position of stents based on the obtainment of images of the lumen of the blood vessel by means of optical coherence tomography. From the images obtained, a three-dimensional reconstruction is made of the contours of the vessel lumen, data are obtained relating to the diameter of the vessel and the blood flow rate, pressure and resistance in order finally to simulate and optimise the length and/or position of the stent.
Lastly, United States Patent Application US2007/0135707 discloses the obtainment of three-dimensional images with which a model of the vessel to be treated can be constructed in order to detect the lesion and its characteristics and simulate the stent to be used and the position at which it will be placed.